This relates to integrated circuits and, more particularly, to integrated circuits that contain memory.
Integrated circuits often contain volatile memory elements. A volatile memory element retains data only so long as the integrated circuit is powered. In the event of power loss, the data in the volatile memory element is lost. Although nonvolatile memory elements such as memory elements based on electrically-erasable programmable read-only memory technology are not subject to data loss in this way, it is often not desirable or possible to fabricate nonvolatile memory elements as part of a given integrated circuit.
As a result, volatile memory elements are often used. For example, static random-access memory (SRAM) chips contain SRAM cells, which are a type of volatile memory element. In programmable logic device integrated circuits, SRAM cells may serve as configuration random access memory (CRAM) cells. Programmable logic devices are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. CRAM cells are used to store configuration data supplied by the user. Once loaded, CRAM cells supply control signals to transistors to configure the transistors to implement the desired logic function.
Volatile memory elements often occupy a significant portion of an integrated circuit and are prone to semiconductor reliability issues such as latch-up. Latch-up is a type of short circuit that can sometime arise between the power supply rails of a memory element. Each memory element includes at least an n-channel transistor coupled to a p-channel transistor, which effectively forms a parasitic silicon-controlled rectifier (SCR) structure in the semiconductor substrate. During a single-event latch-up, heavy ions or protons from cosmic rays or other source of ionizing radiation can strike the SCR, which might cause parasitic bipolar transistors in the SCR to conduct, thereby creating a low resistance path between the positive and ground power supply rails. When this occurs, the latch-up may result in circuit malfunction or potentially destruction of the entire device.
It is within this context that the embodiments described herein arise.